Anime

Japan has a pretty big lead in the animation sector, and their cultural dominance in the space can't be overstated. Billbilli, a pretty major Chinese streaming site, is named after the nickname of a character from a Japanese manga/anime series (Mikoto Misaka from A Certain Scientific Railgun), and a lot of animation studios (and even just solo artists) in other Asian countries end up working as contractors on Japanese productions because there is just so much work there (it certainly doesn't hurt that the pay for these jobs is usually pretty bad for people living in Japan). The Japanese government also actively subsidizes anime because they recognized the benefits to having cultural exports boost their international reputation. I'd imagine China is doing similar things, but they would have started later. Lastly, the companies that get distribution rights and put together the subs and dubs (Crunchyroll et al) all already have agreements with Japanese companies, and not Chinese ones, so even if there were a bunch of high quality donghua, they'd still likely be overlooked in favor of Japanese anime.

As for things being explicitly socialist, China tends not to be particularly evangelistic about socialism or their political system, so it's not surprising that the shows that do get made don't tend to feature them heavily. Also, while I don't know what genres are popular in China, I'd wager they're in a similar place to modern anime where modern or futuristic shows with political leanings where discussions on political economy and the superiority of socialism would be possible have largely died out.

Unfortunately, so far there's only been one season of Chinese animation that I thought was worth watching, and it was Link Click S1, which is hardly socialist.